Scarlet Smoke
by amieofabc
Summary: Sherlock is possessed by a demon bent on destroying him from the inside-out. Prompt from Sherl0ckian, rated T for eventual violence and possible language.
1. Chapter 1

John was out and they hadn't had a case in nearly three weeks. Sherlock was seriously considering arranging a murder so he could stop being so damn _bored. _The cold cases Lestrade had sent him weren't working anymore, they were all too simple. Mrs. Hudson was never exactly the pinnacle of intelligent conversation, and besides, she was visiting Mrs. Turner, and he didn't have any experiments he could line up easily. If not for John, he likely would have shot up by now.

So naturally, when his phone buzzed, he leapt on it.

_Bored, gorgeous? How about hide-and-seek?  
-JM _

He couldn't help but grin, heart racing. Now there was the kind of thrill he'd been craving.

_I need an address.  
-SH_

It came quickly, somewhere abandoned from what he remembered of maps. He snatched John's gun out of its place in his dresser (top left drawer, obvious) before he dashed out for a cab, looking for all the world like a giddy schoolboy.

There was no sign of Moriarty-or, for that matter, any human being-when he arrived at the house, a decrepit establishment. The dust patterns told him nobody had come in for some time, and for a moment he wondered if he'd come to the right place, but the text had said hide-and-seek, after all. For the time being he explored the place, memorizing the structure and the shadows. There was nothing extraordinary to it until one of the shadows moved. There was no source to the motion, unexplainable. Shadows didn't move by themselves.

A fear he didn't understand crawled up his spine. This thing was no more than a whisp of smoke, practically transparent, and yet there was a darkness behind it that made his blood freeze in his veins. It pulsed, swirling in front of him as if it were hesitating. Cautiously, he took a small step forward. The thing froze, not even twitching, then floated a few inches closer.

"What are you?"

It glowed scarlet, then a voice scarcely more than a hiss came into Sherlock's conciousness. It did not speak audibly, rather it was as if a thought not his own had flitted across Sherlock's brain. The thought was but a single word, a whisper- _**Demon.**_

He staggered backwards. "Get out of my head."

The demon laughed, which was a thousand times worse. The gnarled sound rang through every thought. _**You have a fascinating mind. I will enjoy watching it collapse.** _It lunged forward before Sherlock had a chance to react, aiming straight for his chest. Pain ripped through him, spreading outward like a blaze. Was he screaming? He must've been; agony like this couldn't be endured in silence, but the roaring in his ears overtook everything else.

Quickly as it began, the pain ceased, replaced by an icy numbness that seeped through his veins as he lay gasping for oxygen. _**Do not try to fight.**_

It sounded so calm. Infuriatingly so.

"This is Moriarty, isn't it? You don't exist." Sherlock hissed through his teeth.

_**You don't believe in me. That fact does not diminish or invalidate my existence. I am every bit as real as you, and no mortal, not even dear Moriarty, could replicate **_**_this_-**

Another wave of pain, sickening in its intensity, washed over him.

_**Do you see?**_

"Y-yes." he forced out. "What do you want with me?"

_**You are not at liberty to ask questions. But I like an intellectual, they go down the hardest. And you...I can already see that you will not be an easy break. Good.**_

"What do you mean by break?"

_**I am here to prove to you just how insane you really are. You might learn something. And as I've shown you, it is unwise to fight me when I have made up my mind. The first thing you will learn is just how little control you have. **_

The liquid ice in Sherlock's blood seemed to suddenly solidify. There was a moment of dizziness, and, suddenly, he was on his feet. "Alright, you can make me move where you like. Now what?"

_**Now we go home, Sherlock. Don't you want to see John?**_

John. No.

**_You know what happens if you fight me._**

"I'll go where you like, just please leave John out of this."

**_I shall consider it. For now, you will comply or I can guarantee dear John's death by your hands._**

Sherlock swallowed. "Fine."


	2. Chapter 2

He didn't fight the demon on the way back to Baker Street. When John asked him where he'd been and was he alright, he looked pale, he told him he'd needed to get out of the flat before he went insane (well it wasn't a lie), and yes, he was fine, never better, just missing the nicotine a little was all (that, after all, wasn't a lie either). The demon stayed quiet, almost as if it wasn't there, in fact he could've pretended it wasn't but for the icy numbness still permeating his blood and the quiet whispering in his thoughts. None of the words were discernible, but they were deadly, no doubting that.

And constantly, there was the fear sitting in Sherlock's stomach that, any minute, the demon would grow bored of whatever it was doing and unleash itself on John. This thought, in fact, consumed him to the point that he didn't notice how the whispering spread, tingling and pricking over every word. His lips moved, suddenly, of their own accord, he spoke in a voice not his own. "John, this show is dull to the point of being ridiculous." Yes, it was something he would've said had he been paying attention to what John was watching. The demon was covering for him, seeming to think he was being too quiet. That was too much, controlling his speech, and a scream of fury welled inside of him.

The demon, though, saw it coming, and choked it as it rose. The result was a throat spasm and a small, coughing kind of sound. "Well I'm not making you watch it; you can leave if you like. Don't want to lose too many brain cells." John hadn't noticed a thing.

"Fine." He heard himself say, felt himself rise and walk to his room. He gritted his teeth and spoke directly to the demon. "Alright. You can make me say what you like. How did you know what I'd have said?"

**_Memory is a wonderful thing, wouldn't you say?_**

"You…you looked at my memories?"

**_If I can control your nerve endings, motor control, and a few key thought processes, is the ability to access memory really such a far-off thing?_**

Sherlock opened his laptop (of his own accord, thank god) and furiously began typing.

**_Running to Mycroft?_**

He didn't answer.

**_Sherlock, really, I thought you were supposed to be intelligent. Don't tell me you've forgotten how I can hurt you._**

The pain returned, almost fiercer than the first time, and so much worse, the demon wouldn't let him scream, and the anguish built up like bile in his throat—

It lasted perhaps ten seconds. He gasped when it ended, shaking. He remembered that pain from years ago. The demon had tailored his torture with memory, made it so he could experience the sickening nostalgia of a cocaine withdrawal whenever he disobeyed. "Fine." he muttered. "Hurt me all you like." he started to go to bed. "Whatever it is you're trying to do, that won't change a thing." Nothing. "Remind me, what is it you're trying to do?"

_**I have shown you that you do not possess as much control as you wish to believe. That was lesson one. Then I proved that even you cannot write off your own body as "transport", that even you are not immune to pain. Lesson two. **_

"And lesson three?"

_**That you care about all those stupid, fleeting little people more than you know.**_

This demon was growing impatient, both with Sherlock and itself. He was proving a tougher break than even it had anticipated initially. It writhed, swelled with mild frustration, bringing up, again, that fear for John, conjuring an image of the soldier blank-faced and blood-covered and playing it behind Sherlock's eyes.

Through gritted teeth, Sherlock's shout of shock slipped out.

_**Rethink your options.** _

"I still have more than you think."

This was the demon's first snapping point. The next thirty minutes or so are a struggle between the two through a combination of physical pain and a barrage of John, thrown through a window, John shot through the head, John poisoned, John stabbed, John burning, and with the implication, heavy, that it had all been done by Sherlock's hand. It is anguish and the fight is not a silent one this time, Sherlock thrashes as he fights back and ends up on the floor. When at last it lets up, he lies twitching for a few moments. Coughing, he finally sat up, glancing around. Nothing had been moved. John had slept through the attack.

"And what was that supposed to show me? You've seen my mind, you know this won't work."

**_It worked more than you'll admit. But I must _****_apologize, that was a lack of control on my part due to frustration. This-_**_ -_it seized control of Sherlock's movements again, forcing him to stand and creep silently into the kitchen of the flat- _**is your punishment. Your persuasion. **_

Through the haze of exhaustion and ice in his blood, Sherlock felt his fingers curl around the handle of a knife.

"No."

He was already stepping forward towards John's room.


	3. Chapter 3

It was true that Sherlock didn't think much of physical pain or exhaustion. The mind was what mattered, so long as that was healthy nothing else should matter. the messy (if fascinating) business of bones and organs and muscle, that was all just transport, mere tools to aid the mind. At the moment though, he'd have given up several of his favorite books to ease the cramps shooting up is calves as he resisted the demon's push toward's John's room. He hadn't been able to drop the knife without being forced into a position that would have left him with a nasty wound, and both moving backwards and remaining immobile had proven impossible. His defenses against the demon, whatever they might have been, were weakening. At the moment he was moving at the slowest pace he could manage towards a sleeping John.

The moon outside spilled in a shaft out through John's door and glinted off the edge of the weapon curled tight in Sherlock's right hand. In a proper state of mind, Sherlock might have found the image to be an amusing cliche. In the current light, it only made him shudder as he clenched his jaw to stop himself from going into the room. In a fit of desperation, he hoped vehemently that John would have a nightmare, one bad enough for him to wake up and fight Sherlock off before he could be hurt.

Inches away from the headboard, Sherlock knew that this wouldn't be the case. John was having a nightmare, yes. He looked too much like he did awake to be sleeping peacefully, but from the way he was clutching at the sheets and how his eyebrows twitched together, it was the kind that wouldn't relent into wakefulness until morning.

**_Poor dear looks pained. Should we end his nightmare for him?_**

The knife halted mere centimeters from John's throat. Sherlock's heart pounded in his own. He choked down his plea to the demon, because what would it do? It wouldn't listen, and the noise might wake John, and how damaged would they be if he woke up to find Sherlock holding a knife to him? At best, he wold move out. The demon chuckled.

**_Look at that. Blood about to spill and you're still more worried about what he thinks of you than what you're about to do to him._**

It was right. He'd never see John again, but at least he would be alive, at least the separation would be by choice. But a tightening in his throat told him that an attempt to warn him would be useless now. Still, the knife hadn't moved.

_**Listen to your heart pound. Oh, you truly are unused to fear.**_

He didn't think of anything biting to reply with, didn't tell the damn thing to sod off, or even try to move. Because it was true, he didn't know how to experience fear, didn't what it-or he-could do. Adrenaline in his veins, he could handle, he lived for that. But this was heavy, this tied his stomach in knots and would drive him insane. So he closed his eyes and let himself breathe slowly. One thought entered his mind and grew, invading the strength he still had left and keeping him still.

_Don't hurt John._

For nearly three hours the demon kept him there with the knife a breath away from John's skin. He ignored the pain in his legs and the demon's whispering-it seemed to be debating with itself-as best as he could and repeated his mantra until, finally, the muscles in his shoulder relaxed, just barely.

_**Go to** **bed.**_

He obliged, walking stiffly back to his own room. Placing the knife on a shelf and flexing his fingers, he asked simply "Why?"

_**Think of this as a warning for your insolence earlier. Killing him now would only complicate things.**_

Sherlock tried to linger on the word "now", and that it plainly meant "we'll kill him later". "Any chance of me preventing that?"

_**Stop resisting. I did try to ask nicely.**_

And he was so tired. Physically taxed, mentally exhausted, even-dare he think it-emotionally drained. In that haze of weariness and fear, he made a deal with the demon.

"Alright." he said, "Alright, you can have me, you can control everything, my words, my movements, all of it. But if you go after John in any way, I will fight you on it."

_**Agreed.**_


End file.
